We have tried to keep the predicate descriptions clear and concise.
First, the predicate name is printed in bold face, followed by the
arguments in italics. Arguments are preceded by a mode indicator. There
is no complete agreement on mode indicators in the Prolog community. We
use the following definitions:44These
definitions are taken from PlDoc. The current manual has only one mode
declaration per predicate and therefore predicates with mode (+,-) and
(-,+) are described as (?,?). The @-mode is often replaced
by +
| ++ | Argument must be ground, i.e., the argument may not contain a variable anywhere. |
| + | Argument must be fully instantiated to a term that
satisfies the type. This is not necessarily ground, e.g., the
term
[_] is a list, although its only member is
unbound. |
| - | Argument is an output argument. Unless
specified otherwise, output arguments need not to be unbound. For
example, the goal findall(X, Goal, [T]) is good style and
equivalent to findall(X, Goal, Xs), Xs = [T]45The
ISO standard dictates that findall(X, Goal, 1) raises a type_error
exception, breaking this semantic relation. SWI-Prolog does not follow
the standard here. Note that the
determinism specification, e.g., ``det'' only applies if this
argument is unbound. |
| -- | Argument must be unbound. Typically used by predicates that create `something' and return a handle to the created object, such as open/3 which creates a stream. |
| ? | Argument must be bound to a partial term of
the indicated type. Note that a variable is a partial term for any type.
Think of the argument as either input or output or
both input and output. For example, in
stream_property(S, reposition(Bool)), the reposition
part of the term is input and the uninstantiated Bool is
output. |
| : | Argument is a meta-argument. Implies |
| @ | Argument is not further instantiated. Typically used for type tests. |
| ! | Argument contains a mutable structure that may be modified using setarg/3 or nb_setarg/3. |
Referring to a predicate in running text is done using a predicate indicator. The canonical and most generic form of a predicate indicator is a term <module>:<name>/<arity>. If the module is irrelevant (built-in predicate) or can be inferred from the context it is often omitted. Compliant to the ISO standard draft on DCG (see section 4.13), SWI-Prolog also allows for [<module>]:<name>//<arity> to refer to a grammar rule. For all non-negative arity, <name>//<arity> is the same as <name>/<arity>+2, regardless of whether or not the referenced predicate is defined or can be used as a grammar rule. The //-notation can be used in all places that traditionally allow for a predicate indicator, e.g., the module declaration, spy/1, and dynamic/1.